1. Field of the Invention
This invention relates to an optical amplifier, particularly a semiconductor optical amplifier having non-linear characteristics.
2. Related Art
It is known that semiconductors can act as optical amplifiers. When certain semiconductors are subject to an injected electric current, an incident photon causes an electron to traverse semiconductor's gap band with the result that an additional photon is generated thereby producing light amplification. Semiconductor optical amplifiers which operate in this way are well known and reference is directed to "Long Wavelength Semiconductor Lasers" G. P. Agrawal and N. K. Dutta; Van Nostrand, Chapters 1 to 6.
The semiconductor material which is used as the active amplification region of the device may comprise a bulk material or for example, a stack of multiple quantum wells (MQW). A problem with the semiconductor material that is used for the amplifications region is that it suffers from a gain-saturation effect which may be produced by amplified spontaneous emission (ASE). For this reason, typical devices that use bulk semiconductor material in a parallel sided channel, usually have a length of 500 microns or less and a width of 1-2 microns, because if the device were to be made longer, there would be no improvement in gain. For MQW devices, the gain per unit length is slightly lower than for bulk material devices, and amplifiers of length up to 1 mm have been produced hitherto but it is has been considered that devices of longer length would suffer from ASE, with no improvement in gain. Longer, tapered devices have been reported, of length 1-3 mm, in which tapering of the amplifier is arranged to offset partially the onset of gain saturation. Reference is directed to S. El Yumin et al, "Taper Shape Dependence of Tapered-Waveguide Travelling Wave Semiconductor Amplifier", IEICE Transactions on Electronics, Vol. e77, No 4, April 1994, Tokyo Japan. Reference is also directed to D. Mehuys et al, "11.6 W Peak power diffraction limited diode-to-diode optical amplifier" Appl. Phy. Lett. Vol. 62, No 6, Feb. 8, 1993, pp 544-546, which discloses a broad area travelling wave amplifier of width 600 .mu.m and length 2200 .mu.m.
Another disadvantage of longer devices is that they are harder to mount. Conventionally, SOAs are mounted using headers designed for laser diodes, which tend to be shorter and as a result, it is not straightforward to package longer devices.
Also, longer devices consume more power, so that it has been considered disadvantageous for the device to be longer than that at which optical saturation occurs.
Semiconductor optical amplifier devices can be used for a number of different purposes and a review is given in K. E. Stubkjaer et al, "Optical Wavelength Converters", Proc. ECOC '94, pp 635-642. SOAs can be used as modulators, in which an optical signal, modulated at a given bit rate, is fed into the amplifier, together with a separate target wave. The modulated signal produces gain-saturation for successive bits and as a result, the target wave is modulated with the input bit pattern. This is known as cross gain modulation (XGM). The modulation may also produce a phase shift in the target wave and this is known as cross phase modulation (XPM). Both of these processes may produce wavelength conversion. For example, the target wave source may be at a different wavelength to the modulated input source so that the bit modulation is transferred from the input optical source at a first wavelength to the target wave at a second different wavelength.
In order for the modulator to be effective, for example in an optical data transmission network, it is desirable that the amplifier exhibit uniform amplification characteristics over a wide range of bit modulation frequencies. For example, Stubkjaer supra suggests a bit rate transparency to more than 5-10 G-bit/s. A bit rate of 20 G-bit/s has been reported by J. M. Wiesenfeld, J. S. Perino, A. H. Gnauk and B. Glance, "Bit Error Rate Performance for Wavelength Conversion at 20 G-Bit/s", Electron. Lett. 30, pp 720-721 (1994) although it is not clear from Wiesenfeld et al whether the modulator was operating within a 3 db bandwidth.
Hitherto, it had been considered that the bandwidth was limited by the differential carrier recombination rate in the amplifier, this rate including spontaneous emission and stimulated emission.
However, in accordance with the present invention, it has been found that the -3 db bandwidth of the gain of the amplifier in respect of the bit modulation rate, is a function of the length of the path through the amplifier. Thus, in accordance with the invention, it has been appreciated that by increasing the length of the path, this bandwidth can be increased.
Semiconductor amplifiers can also be used to produce wavelength conversion by a different process known as four wave mixing. This is discussed in Stubkjaer supra and a fuller theoretical discussion is given in "Population pulsations and nondegenerate four-wave mixing in semiconductor lasers and amplifiers" G. P. Agrawal, J. Opt. Soc. Am. B, Vol 5, No 1, January 1988 pp 147-159. In four wave mixing, pump radiation at a pump wavelength .lambda..sub.p is fed into a semiconductor amplifier, together with an input signal .lambda..sub.i of a different wavelength to the pump signal. In a typical example, the pump waveform has an energy of 10 mw whereas the input signal has an energy of 1 mw. The wavelength of the input signal is close to that of the pump, typically with a wavelength difference of .congruent.2 nm. The two beams are of the same polarisation and consequently beat coherently, with a beat frequency in this example of .congruent.100 GHz. The resultant beat waveform causes the carrier density in the amplifier to oscillate. This produces a non-linear effect on the gain, which lags the input waveform and beats with it. It can be shown that this produces a wavelength converted signal .lambda..sub.c, with a wavelength .lambda..sub.c =2.lambda..sub.p -.lambda..sub.i. The converted signal .lambda..sub.c and the input signal .lambda..sub.i are equally spaced in terms of wavelength above and below the pump wavelength .lambda..sub.p.
Four wave mixing has the advantage that the conversion process is extremely fast as it does not rely on carrier recombination as in XGM and XPM. Furthermore, there is less distortion but four wave mixing suffers from the disadvantage that the converted signal is of low power and the signal to noise ratio can be a problem in respect of the converted signal.
However, in accordance with the invention, it has been found that the conversion efficiency for four wave mixing is function of the length of the path through the amplifier. Thus, in accordance with the invention, it has been appreciated that by increasing the length of the path, the four wave mixing efficiency can be increased.